In recent years, a water-absorbing resin that is one example of a hydrophilic polymer has been widely used in various applications, such as sanitary goods (disposable diapers, sanitary napkins, adult incontinence products, etc.) and humectants for soil, and the water-absorbing resin has been produced and consumed massively. Especially, in the case of the application of the sanitary goods, such as disposable diapers, sanitary napkins and adult incontinence products, the amount of water-absorbing resin used tends to be increased whereas the amount of pulp fiber tends to be reduced for the purpose of reducing the thickness of the product. Here, there is a demand for the water-absorbing resin having a high absorption capacity under load and meanwhile there is a demand for a low-cost manufacturing since the amount of water-absorbing resin used for each sanitary goods is large. Therefore, there is a demand for a reduction in an energy consumption at a manufacturing line of the water-absorbing resin, a reduction in the amount of wastes, and an establishment of a rational manufacturing method realized by these reductions.
As desired properties of the water-absorbing resin, there are a high absorption capacity, a high absorption capacity under load, a high absorption rate, a small amount of residual monomer, a small amount of water extractable polymer. Among these properties, the improvement of the absorption rate is one of the basic objects regarding the water-absorbing resin, and many methods for improving the absorption rate have already been proposed.
The absorption rate depends largely on a specific surface area. Therefore, in a method for reducing a particle diameter, there are problems of deterioration of other physical properties, such as the generation of dust and deterioration of liquid permeability. Here, many methods, such as foaming and agglomerating, for improving the rate have been proposed, and known methods are, for example, (i) a method for dispersing solid matters (solids) and carrying out foaming polymerization (Document 1), (ii) a method for dispersing gas and carrying out foaming polymerization (Document 2), (iii) a method for carrying out foaming polymerization using carbonate (Documents 3 and 4), (iv) a method for carrying out foaming polymerization using 0.1% to 20% of a surfactant (Documents 5 and 6), (v) a method for agglomerating water-absorbing resin by extruding them from an aspheric die (Document 7), (vi) a method for agglomerating water-absorbing resin fine particles using a polyvalent metal (Document 8), (vii) a method for heating water-absorbing resin using a microwave (Document 9), and (viii) a method for incorporating a filler (Document 10).
Moreover, another known method is a method for polymerizing, by carrying out foaming polymerization, a slurry of minute precipitate of sodium salt of acrylic acid, containing micro bubbles of an inactive gas (Document 11).
In a method for improving the absorption rate by increasing the surface area by reducing the particle diameter, the increase in the amount of fine powder causes the deterioration of other physical properties. Moreover, in a method for agglomerating the water-absorbing resins to improve the absorption rate, its process is complicated. In addition, since the intensity of the agglomerating is generally low, there are problems, such as the generation of dust and deterioration of other physical properties. Further, in a method of the foaming, an additional foaming agent, surfactant and/or the like are required, and in addition, there are problems, such as (i) an increase in cost of transportation and warehousing due to deterioration of a bulk specific gravity by the foaming and (ii) the generation of fine powder and deterioration of physical properties since foamed particles are weak. Further, in the foaming polymerization, there are problems, such as an increase in the amount of residual monomer and a difficulty of improving the absorption capacity under load (AAP, Absorbency Against Pressure).
Moreover, in the case of conventional water-absorbing resins, it is difficult or impossible to control, within respective preferable ranges, all of the physical properties, such as a water-absorbing rate, the absorption capacity under load, the particle size and the amount of residual monomer. In addition, even if the water-absorbing resin whose all physical properties are controlled within respective preferable ranges is obtained and in the case of manufacturing diapers on a large scale using this water-absorbing resin, the diaper may not provide its performance which has been provided in an experimental laboratory.
As a result of a study on the reason why the diaper does not provide its performance, it is found that in a case where the water-absorbing rate is improved (when foaming or agglomerating is carried out), the water-absorbing resin is damaged when it is transported, for example, on a line at a factory, and the fine powder is easily generated, and therefore, for example, the performance of the diaper deteriorates when the water-absorbing resin is used in the diaper.
On this account, there is a need for the water-absorbing resin whose performance is improved so that the diaper can perform maximally. Specifically, there is a need for the water-absorbing resin whose water-absorbing rate is high and which is not easily damaged (which does not generate the fine powder easily).    [Document 1] U.S. Pat. No. 5,985,944 (published on Nov. 16, 1999)    [Document 2] U.S. Pat. No. 6,107,358 (published on Aug. 22, 2000)    [Document 3] U.S. Pat. No. 5,712,316 (published on Jan. 27, 1998)    [Document 4] U.S. Pat. No. 5,462,972 (published on Oct. 31, 1995)    [Document 5] U.S. Pat. No. 6,136,973 (published on Oct. 24, 2000)    [Document 6] U.S. Pat. No. 6,174,929 (published on Jan. 26, 2001)    [Document 7] U.S. Pat. No. 6,133,193 (published on Oct. 17, 2000)    [Document 8] U.S. Pat. No. 5,002,986 (published on Mar. 26, 1991)    [Document 9] U.S. Pat. No. 6,076,277 (published on Jun. 20, 2000)    [Document 10] U.S. Pat. No. 6,284,362 (published on Sep. 4, 2001)    [Document 11] Japanese Unexamined Patent Publication No. 115313/1991 (Tokukaihei 3-115313 (published on May 16, 1991))